The present invention relates to gas monitoring devices, and more particularly to methods and apparatus for detecting and identifying various gas constituents in a sample gas stream and/or determining the concentrations of such constituents.
The detection of gases and vapors at low concentrations is often difficult due to limitations in the sensitivity of detector devices and measurement instruments. The process of detecting various constituents within a gas sample at low concentrations can be greatly enhanced if the constituents can be concentrated prior to detection. One approach for concentrating selected constituent gases is described in xe2x80x9cQuartz Crystal Gas Monitor With Gas Concentrating Stagexe2x80x9d, Kindlund et al, Sensors and Actuators, 6 (1984) pp. 1-17. Kindlund et al. suggest providing a gas concentrator in front of a detector to increase the concentration of the desired gas constituents at the detector. The gas concentrator of Kindlund et al. includes a thick organic sorbent layer that is coated on the walls of a cavity. When cool, the sorbent layer adsorbs the desired gas constituents from the gas sample flowing through the cavity. A heating pulse is then applied to the sorbent layer, causing the adsorbed constituents to desorbs into the cavity to produce a short concentration pulse. The concentration pulse is conducted to a quartz crystal gas monitor that ultimately registers the presence of the constituent.
A limitation of Kindlund et al. is that typically sorbent materials can only accumulate a limited amount of gas constituents. Thus, the concentration pulse produced when the sorbent layer is heated is also limited, thereby limiting the effective sensitivity of the detector. What would be desirable, therefore, is a concentrator and/or sensor assembly that can further increase the concentration level of desired gas constituents at the detector to produce a detector of increased effective sensitivity.
The present invention overcomes many of the disadvantages associated with the prior art by providing a concentrator and sensor assembly that use phased heaters to increase or multiply the concentration levels beyond those that can be achieved by a single interactive element having a sorbent material. Generally, this is accomplished by providing two or more interactive elements that are selectively heated in a time phased sequence so that each of the interactive elements becomes heated and desorbs constituent gases into the sample fluid stream at substantially the time that an upstream concentration pulse, produced by heating one or more upstream interactive elements, reaches the interactive element. As can be seen, this produces a multiplication effect that can significantly increase the concentration of the gas constituents at the detector, thereby increasing the effective sensitivity of the detector.
In a first illustrative embodiment, a concentrator is provided for concentrating one or more constituents in a sample fluid stream. The concentrator preferably has two or more interactive elements spaced along and exposed to the sample fluid stream. Each of the interactive elements include an interactive substance that adsorbs and desorbs selected constituents of the sample fluid stream, depending on the temperature of the interactive element. Two or more heater elements are provided, with each heater element in thermal communication with a corresponding interactive element.
A controller energizes the heater elements in a time phased sequence. The controller preferably energizes the heater elements such that each of the corresponding interactive elements become heated and desorb selected constituents into the sample fluid stream at substantially the time at which an upstream concentration pulse, produced by one or more upstream interactive elements, reaches the interactive element. It is contemplated that a large number, N, of interactive elements may be used to achieve the desired multiplication of concentration of constituent gases in the concentration pulse by a factor N.
The resulting concentration pulse may then be provided directly to a detector for detection and analysis. The detector may be a thermal conductivity detector, discharge ionization detector, or any other type of detector such as those commonly used in gas chromatography. More preferably, however, the resulting concentration pulse is first provided to a separator. The separator separates selected gas constituents of the resulting concentration pulse into individual constituent components. The detector may then detect the concentration of each constituent that elutes from the separator.
The heater elements are preferably formed from a resistive material having a common resistance and length along the flow direction. As such, the controller can equally energize the heater elements by providing an equal voltage, current, or power pulse to each heater element. The voltage, current, or power pulse may have any desired shape including a triangular shape, a square shape, a bell shape, or any other shape. The shape or height of the voltage, current, or power pulse may even be chosen to produce a temperature profile that only desorbs selected gas constituents from the sorbent material.
It is also contemplated that the length of the heater elements may increase along the sample fluid stream. The length of each heater element may be increased, relative to the upstream heater elements, by an amount that corresponds to the expected increased length of the concentration pulse of the upstream heater elements caused by diffusion. To match this diffusion effect for best utilization of the growing concentration wave in the concentrator, the length of each of the heater elements may be similarly increased to produce the same resistance, thereby tailoring equal voltage, current, or power pulses to be used for each heater element to achieve equal temperature profiles. Alternatively, all heater elements may have the same length as the N-th element, so that the controller may provide equal voltage, current, or power pulses, suitably phased in time, to all heater elements to result in equal temperature profiles.
It is also contemplated that the two or more interactive elements need not be separate elements, but rather may be formed from a single interactive layer. Two or more heater elements may then be in thermal communication with different portions of the interactive layer. This configuration may simplify the manufacture of the concentrator.
The present invention also contemplated a number of methods. In one illustrative method, a sample fluid flow or stream is provided using a pump, thermal convection, or the like. The sample fluid stream is allowed to pass over two or more interactive elements (or an interactive layer) until the interactive elements adsorb one or more constituents from the sample fluid stream and reach equilibrium. Thereafter, the two or more interactive elements are heated in a time phased sequence.
Preferably, an upstream interactive element is first heated, which causes the upstream interactive element to increase in temperature and to desorb selected constituents into the sample fluid stream to produce a first concentration pulse that is carried by the sample fluid stream downstream toward a downstream interactive element. Thereafter, the downstream interactive element is heated as the first concentration pulse reaches the downstream interactive element. This causes the downstream interactive element to desorb selected constituents into the sample fluid stream and at least partially overlap the first concentration pulse to produce a larger concentration pulse that is carried by the sample fluid stream further downstream. The larger concentration pulse has an increased concentration level of the selected constituents than that of the first or second concentration pulses. It is contemplated that any number of downstream interactive elements may be heated in a like manner to produce an even further increased concentration level at the output of the concentrator.
After the concentrator provides a desired concentration pulse, selected constituents may be separated to provide one or more individual constituent components. The concentration of the individual constituent components may then be sensed and analyzed as desired.